The final lines "I took the one less traveled by / And that has made all the difference" are often cited as emblematic of America's individualist spirit of adventure, in a reading that assumes they are to be taken literally. But whatever difference the choice might have made, it was not made on the basis of a discerned difference between the two paths that opened up before the traveller. The speaker admits in the second and third stanzas that both paths may be equally worn and equally leaf-covered, and it is only in his future recollection that he will call one of the two roads, the one he took, "less traveled by."
The "sigh" can be interpreted as one of regret or of self-satisfaction; in either case, the irony lies in the distance between what the speaker has just told us about the roads' similarity and what his or her later claims will be...
...the poem is about "the human tendency to wobble illogically in decision and later to assume that the decision was, after all, logical and enormously important, but forever to tell of it 'with a sigh' as depriving the speaker of who-knows-what interesting experience."